1. Field of the Invention
The present invention relates to methods of detecting changes in the occupancy state of a portion of route suitable for being traveled along by objects following its axis in a given scene, e.g. for the purpose of evaluating variations in traffic density on the portion of route, methods which find a particularly advantageous application in the field of detecting incidents of any kind that might arise on the portion of route.
2. Description of the Related Art
Traffic, in particular motor vehicle traffic, has been increasing continuously over several years, and in certain circumstances, e.g. following an incident on the road used by such vehicles, congestion occurs which undoubtedly impedes traffic flow. Proposals have therefore been made to remedy such drawbacks by detecting any incident that occurs on a portion of route (in this case a portion of road) as quickly as possible after it has occurred, and then controlling and modulating vehicle traffic on said portion of road, and regardless of whether the portion of road is used by many vehicles (a traffic lane) or by few vehicles (emergency stop lane, a zebra zone, a refuge, etc.).
In order to be able to detect such incidents, it is necessary to provide sensors capable of giving an image of vehicle traffic on a portion of road. Numerous sensors have been developed. For example, a sensor has been devised comprising photosensitive receivers associated with light rays directed towards the roads along which vehicles are traveling and returned by reflecting surfaces disposed for this purpose on the roadways, with the photosensitive receivers outputting signals each time a vehicle interrupts the light beams.
That technique gives good results. However the signals delivered are representative of traffic at a determined point only, and the sensors used are not flexible in use, since they require elements to be applied to the roadway at locations that are well defined, and to ensure that said reflecting surfaces reflect continuously by also providing artificial illumination when the lighting of the scene is low. Such elements therefore cannot be moved without difficulty, and once they have been put into place, they require frequent intervention, if only to keep their reflecting surfaces clean.
Other sensors have been made for increasing the area under surveillance. This applies to a sensor constituted by a magnetic loop embedded in the roadway. Such a sensor mitigates some of the above-mentioned drawbacks, but it remains too geographically restricted in use, specifically because it remains associated with a determined location of the roadway and requires major roadworks for installation by sawing into the roadway.
A device has also been developed for implementing the method described in EP-A-0 277 050. In that method, a main real image is initially formed of the portion of road in a plane that forms a non-zero angle with said portion of road. This main image is then subdivided into a plurality of points, and the relationship is determined between the size of a unit length taken substantially at the portion of road and the size of its image formed in the main image, as a function of the number of points covered by the image and the location of the unit length on the portion of road. A secondary image is also determined in the main image, the secondary image corresponding to a longitudinal mark associated with the vehicle on the portion of road, the different successive positions of the secondary image being defined by correlations with the number of points covered by said secondary image, it being understood that said secondary image in said relationship corresponds to a constant length on the portion of road.
The device described in that prior document gives very good results and makes it possible to determine a very large number of parameters defining traffic density on a portion of road. Nevertheless, it is very expensive or too complex for certain applications, thereby restricting use thereof.
A simpler device has also been developed such as that described in U.S. Pat. No. 4,258,351. That device comprises a series of photosensitive cells distributed in the focal plane of a converging lens. Each cell is constituted by a strip, and each strip is designed so that its length is equal to the width of the image of the road formed by the lens. Said length thus complies with the perspective relationship for the road.
That technique presents the advantage of being easy to implement, but it also presents drawbacks: it requires an implementation for each road and only one signal is obtained by lines crossing the road, thus making the signals very difficult to interpret.
Other devices have been developed that give good results, constituted by a video camera having a target constituting an optoelectronic converter of an optical image, said target being controlled by a programmable processor member.
By way of example, such a device is described in FR-A-2 679 682, which discloses an implementation of a method enabling an incident to be detected on a portion of route situated in a scene when said portion of route is suitable for having objects traveling therealong.
Such a device presents advantages over the prior devices. In addition to being made out of elements that are commonly available, it enables the images of the portion of route under surveillance to be stored in a memory, where such images can be used subsequently, e.g. to determine the cause of an incident or the like that has occurred on said portion of route.
In addition, in order to be better aware of the nature and the immediate consequences of an incident, thus making it possible to study the cause thereof better, it is possible to modify at will the field of the objective lens of the camera when it is constituted by a zoom lens, and/or to modify the direction in which the optical axis of the camera is pointing by mounting the camera to co-operate with a pointer member so that the direction of its optical axis can be varied in elevation and in azimuth.
These facilities made possible by present video cameras are most advantageous for the operators of traffic routes, in particular roads, but they make it considerably more complicated to implement the method of the kind given in FR-A-2 679 682 for detecting an incident using the technique referred to as “AID”.
The AID technique of Automatic Incident Detection on a portion of route can be implemented only if the image of the portion of route formed on the photosensitive target of the camera is stable for several seconds or even several minutes, which it the time needed by the processor member to execute the program for implementing the method. The method used in that technique requires a manual calibration stage on a stable image. In general, the camera is held stationary and said stage is performed when the device is put into operation. For example, maintenance operations on the camera make it necessary on each occasion to verify that the sensor is properly calibrated.
The method can therefore no longer be implemented when, for example, the direction of the optical axis of the camera changes in elevation and/or azimuth, and/or when the field of the objective lens of the camera is varied, e.g. by zooming into a particular area of the portion of route and/or the scene that includes said portion of route.
The present invention thus seeks to implement a method which makes it possible automatically to detect an incident that has occurred on a portion of route, e.g. using the AID technique described in FR-A-2 679 682, even when the field of the camera lens has been modified, e.g. by zooming, and/or when the direction of the optical axis of the camera has been changed in elevation and/or azimuth, and to do this without making it necessary for technicians to intervene manually after each such modification, for example, while also making it possible to use the devices for implementing prior art methods without needing to add additional hardware means thereto.